As electronic instruments have been miniaturized and improved in performance, the density of printed wiring boards used therein has increased due to high multilayering, thinning and decreases in through-hole diameter and in the distance between the through-holes. Printed wiring boards are also used in recent semiconductor packages of plastic pin grid array or plastic ball grid array wherein semiconductor tips are mounted directly on printed wiring boards and sealed with resins. In the course of the production of such semiconductor packages, printed wiring boards experience high temperatures of 175.degree. C. or higher during wire bonding or resin sealing. So printed wiring boards of low strength and low elastic modulus may cause disconnection of bonding wires or bow or twist after resin sealing. To improve the high temperature characteristics at 175.degree. C. or higher, including strength and elastic modulus, printed wiring boards should have a higher Tg (glass transition temperature). Printed wiring boards for semiconductor packages have growing requirement for higher wiring density, and insulation reliability is an important property.
To meet these requirements, development work on new materials for printed wiring boards is proceeding. As disclosed in Japanese Patent Application Unexamined Publication No. 60-155453, as a means for increasing Tg, curing of polyfunctional epoxy resins with dicyandiamide is studied extensively. Unfortunately the epoxy resins cured with dicyandiamide have a high hygroscopicity and hardly satisfy the high insulation reliability necessary for the future printed wiring boards with higher density. Among various insulation troubles, metal migration (electrolytic corrosion) has been found to be a serious trouble. Metal migration is the phenomenon that the metal constructing wiring, circuit pattern or electrodes on or inside insulation materials migrates on or inside the insulation materials by the development of potential difference under the circumstances of high humidity.
To the contrary, printed wiring boards produced by using polyfunctional phenols as curing agents have a low hygroscopicity and a good resistance against electrolytic corrosion. Some polyfunctional phenols, however, discolor the printed wiring boards during heating processes. To prevent the color change due to heating, in Japanese Patent Application Examined Publication No. 62-28168 is proposed a curing agent which contains a high-ortho phenol-formaldehyde resin derived mainly from phenol or bisphenol A. Even this curing agent cannot provide printed wiring boards having a high Tg enough to tolerate high temperatures of 175.degree. C. or higher.
In the circumstances, an object of the present invention is to provide epoxy resin compositions which are useful for the production of printed wiring boards having a low hygroscopicity, a high heat resistance, good high-temperature characteristics, a high resistance against electrolytic corrosion, a high resistance against color change due to heating and a high Tg.
Another object of the present invention is to provide laminates for printed wiring boards, which are produced by using the epoxy resin compositions and have a high heat resistance, good high-temperature characteristics, a high resistance against electrolytic corrosion, a high resistance against color change due to heating and a high Tg.